1. Under the conditions of these experiments, fluid cultures of H. pertussis have proved less virulent for mice than B-G. cultures. As on B-G. medium (Gray 1946) mouse virulence in fluid medium decreases with increasing physiological age, though less rapidly than on B-G. medium.

  2. Vaccines made from fluid cultures have proved less potent for mice than B-G. vaccines. When cultures of more than 72 hours' incubation are used for vaccine production, they are of little value as immunising agents.

  3. When fluid cultures are agitated continuously during the incubation, the yield is greatly increased. Marked alterations occur, however, in viable counts, in virulence and in immunising potency. The effects on potency are inconstant and not yet completely understood.

  4. The ratio of viable to total counts in standardised suspensions from solid or fluid cultures bears no constant relationship to either virulence by the intranasal route, or to immunsiing potency.

  5. An observation, intriguing in the light of current concepts of growth dynamics, was the common occurrence of lower viable counts at 48 hours than at either 24 hours or 72 hours in suspensions standardised to the same density.

  6. There appears to be little to choose between phenol, formalin, merthiolate and mild heat in producing H. pertussis vaccine.

  7. There is no evidence of autolytic loss of antigenic potency over a period of three months' storage in phenolised vaccine.

  8. No more efficient vaccine for mice, under the experimental conditions employed, has yet been evolved than one prepared from a 24 hr culture on B-G. medium of H. pertussis phase I.

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This work was assisted by a grant from the National Health and Medical Research Council, Canberra F.C.T.

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